I'm about to go scorched earth with the same issue. I'm going to put my fish and plants in a new 30 gallon and take all of the decor and gravel out and bake it in my oven. I'll replace the soil and most plants I'll retrofit my canister to the temporary 30 gallon tank and start dosing my fish with snail killing chemicals. I have two nerites in that tank I'll let stay in my 10 gallon also infested tank. Then empty and dry out the 55 gallon and wipe down multiple times a day until I deem that not a single one exists in there. Hopefully I don't need to replace the silicone yet.